However, to improve the network capacity further, the deployment of more distributed synchronized bss requires high cost and power consumption in the cell-free network. Fortunately, the emerging new technique called ris is able to provide an energy-efficient alternative to enhance the network capacity. Equipped with a low-cost, energy-efficient and high-gain metasurface, ris is becoming a promising smart radio technique for future 6G communications [1]}. With a large number of low-cost passive elements, ris is able to reflect the elertromagentic incident signals to any directions with extra high array gains by adjusting the phase shifts of its elements [2]}. Since the wireless environment can be effectively manipulated with low cost and energy consumption [3]}, ris can be used to improve the channel capacity [4]}, reduce the transmit power [5]}, enhance the transmission reliability [6]}, and enlarge the wireless coverage [7]}.
